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Advanced 2D Optical Lithography Simulator

Optolith™ is a powerful non-planar 2D lithography simulator that models all
aspects of modern deep sub-micron lithography: imaging, exposure, photoresist
bake, development and reflow. Optolith provides a fast and accurate alternative
to experimental evaluation of mask printability and process control. Optolith
simulates both projection imaging and proximity printing with large mask-to-resist
gap. Optolith is fully interfaced to all commercial IC layout tools conforming
to GDSII and CIF formats, as well as to a special proprietary format used by
MaskViews. As part of the Athena process simulation framework, Optolith provides
seamless integration with diffusion, oxidation, implantation, etching, and
deposition simulation capabilities. This simulation environment allows to analyze
complex lithography effects in multilayer non-planar structures generated by
oxidation, deposition and etching. It also provides unique opportunity to investigate
and optimize implantation and etching process steps with real photoresist mask
shapes taken into account.

Automated large-scale experimentation capabilities are available within
the Virtual Wafer Fab

Complete Photolithography Process

Intensity distribution

PAC distribution

Development profile

Imaging of Complex Masks with Various Transparencies and Phase Shifts

The two plots show MaskViews visualization of GDSII layouts with
regular (hexagons) and irregular shapes. These are test layouts for an
LCD application. The Optolith interface with MaskViews allows specification
of variable transmittance and phase shift. In this case, the central octagons
have 100% transmittance while small regular and irregular spots are of
19% transmittance and 45 degrees phase shifts.

The two plots show
corresponding 2D aerial images calculated by projection imaging module
of Optolith (insets are 3D views of the central octagonal spot).

Exposure and Development Simulation in Non-Planar Structures

Optolith has a great advantage in comparison with other commercial lithography
simulators because it can accurately handle non-planar structures. First of
all, seamless integration with the SSuprem and Elite modules allows the use
of representative non-planar substrate geometries. Secondly, simulation of
resist planarization using reflow process gives realistic shapes of resist
layer. Thirdly, the Beam Propagation Method, (BPM), accurately accounts for diffraction
and multiple reflection effects in all non-planar resist layers with arbitrary
shapes. Moreover BPM, takes into consideration local changes of resist refraction
properties with absorbed light energy. This example demonstrates these non-planar
capabilities and particularly the effect of exposure dose on the resist optical
properties. The figures above show intensity distributions in a non-planar
structure (two upper plots) and corresponding developed resist profiles (two
lower plots). The two figures on the left correspond to the case of constant
photoresist refraction index. The two figures on the right correspond to the
photoresist with refraction index varying linearly with the accumulated dose
during exposure process. The comparison of final structures shows that the
dose effect could be very pronounced and should be taken into account. The
strong reflection from the slopped walls with subsequent undercut during development
may even result in complete removal of the resist feature.

Process Control - Smile Plots

Complete lithography simulation from imaging to resist development is the
only practical methodology of process control. One of the key elements of process
reproducibility is the depth of focus (DOF) control. Even after resist planariztaion
the DOF could be different in various areas of the layout due to topological
variations of the preceding process steps. In some cases even small changes
in DOF or other process parameters may result in unacceptable violations of
critical dimensions (CDs). The main controlling parameter for this CD vs. DOF
effect is the exposure dose. Therefore, the only way to characterize the process
response is to vary simultaneously defocus and exposure dose and extract corresponding
CDs. Optolith together with Silvaco Interactive Tools provides an ideal environment
for lithography process control. The image of Deckbuild in the upper left corner
shows a very simple setting for the DBInternal module. It consists of the nested
loop over “dose” and “defocus” parameters of the Optolith
template input deck below. Deckbuild automatically runs the Optolith simulation
77 times with the defocus parameter varying between –1 and 1, and the
exposure dose parameter varying between 100 and 160. All final resist profiles
are saved. Also, the extracted dose-exposure matrix of CD values is stored
in the ASCII file “smile.dat”. Just 36 out of 77 resist profiles
are shown in the “matrix” plot above. It is clear that the second
row, which corresponds to the dose=120 mJ/cm2 and defocus from 0 to 1 micron,
demonstrates the smallest CD variation. TonyPlot can be used for visualization
of the data stored in the “smile.dat” file. The corresponding smile
(or Bossung) plot is shown in the figure to the right.

Full Lithography Process Simulation

These pictures show the effect of defocus on CD variation and resist
shape in comparison with measured resist profiles (SEM pictures were provided
by LG Semiconductor). Several process parameters including bake diffusion
coefficient and Dill’s development coefficient have been calibrated.
It is clearly seen that complex CD extraction, which takes into account
characteristics of the resist shape is necessary to achieve accurate calibration
and optimization of the complete lithography process.

Proximity Printing

Proximity printing or imaging without any reduction lens regained its popularity
recently because it appeared to be a very cost effective solution for active-matrix
liquid crystal display (AMLCD) technology. The accurate simulation of proximity
printing could not be achieved by simplification of “standard” projection
technology methods. Therefore, a separate proximity printing module is implemented
in Optolith. It is seamlessly integrated with other lithography simulation
modules as well as with MaskViews.

The picture on the top demonstrates proximity printing image of circular
feature with radius 15 microns. This is a typical feature size for AMLCD.
This image is calculated for the distance (gap) of 150 microns between
mask and resist film. The single i-line illumination was used. The multi-image
and multi-exposure capabilities of Optolith allows the approximation of
broadband illumination by combining results of several image/exposure simulations
for main lines of the white spectrum. The variation in intensity is typical
for proximity imaging. In the case of broadband illumination these variations
slightly decrease.

The plots show results of subsequent
lithography steps: after exposure and bake the negative resist has been
developed. The developed resist demonstrates a typical ”ripple” pattern
(plot on the top). The post development bake/reflow removes the pattern.